In recent years, audio or video analog signals are frequently processed by sampling, quantization and encoding into so-called pulse code modualtionor PCM signals before their transmission or recording and/or reproduction.
When transmitting or recording and/or reproducing these PCM signals, the practice commonly adopted for obtaining a bandwidth of, for example, 20 kHz and an S/N ratio of higher than about 90 dB, is to use a sampling frequency fs of 44.1 kHz and liner quantization at the rate of 16 bits per word. In this case, however, the rate of transmission is as high as 700 KBPS, or 700 k bits per second.
It is noted that, in digital signals obtained by A/D conversion of analog signals, such as audio or video signals, it is possible to compress the amount of information by taking advantage of the fact that these digital signals are prone to deviation from the statistic point of view and certain portions thereof are less critical insofar as the visual sense and/or accoustic sense is concerned. Thus it is known for example that deterioration in the signal quality occurs on very rare occasions even when these digital signals are subjected to differential or summation processing or to compression and expanding (companding) operations.
With this in view, it has been proposed in our copending Japanese Patent Application Nos. 97687 to 97689/1983, 163054/1983, 166267/1983 and 210382/1983, corresponding to U.S. application No. 700,817, filed 1/31/1985 (now abandoned), that the digital PCM signals, for example, are formed into a block at a predetermined time interval or every after a predetermined number of words and the signals of each block are subjected to predicting or estimating such as differential processing or companding before transmission or recording and/or reproduction.
According to these publications, reference data of at least one word, such as straight PCM data, are provided in each block. The differential data, for example, are sequentially summed or otherwise processed on the basis of these reference data so as to permit complete reconstruction of the original sampling data or straight PCM data in the block. The objective is mainly to minimize error propagation that is caused in general when transmitting differential processing data and to prevent error such as overshoot at the block boundaries from occurring when switching the estimating or companding operations for each block.
When an extremely high bit compression rate is used, e.g. when the 16-bit-per-word data is compressed to 4-bit-per-word data, each word of the reference data corresponds to 4 words of the data obtained after compression. Therefore, when it is desired to elevate the transmission or recording efficiency, it is preferred to effect data transmission or recording and/or reproduction without inclusion of the aforementioned reference data in the block.
However, the S/N ratio is not improved sufficiently when simply transmitting the differential output, or the unusual sound may be produced on occurrence of code error on the transmission channel or during recording and/or reproduction.
With this in views, it is an object of the present invention to provide a digital signal transmission device wherein deterioration of the S/N ratio or production of unusual sounds can be prevented while the transmission bit rate can be reduced to an extremely low level.